Check valves are well known for use in assuring that a flow through a conduit occurs only in a predefined direction. Check valves are used, for example, in backflow prevention assemblies to prevent backflow of one fluid body into another. Backflow prevention is often used in connection with protecting potable water supplies from contaminants which could otherwise be introduced into it via back-siphonage or back-pressure. Many backflow preventors are designed to accommodate pressure commonly encountered in municipal water supplies, such as 150 psi (1,030 kPa) or more.
Several factors are important in designing or selecting a backflow preventor for a particular use, including performance (e.g., minimizing pressure drop), serviceability, and ease and cost of installation. In previous devices, maximizing serviceability has been incompatible with also maximizing the performance and installation factors. Thus, in past devices, efforts to increase the performance and ease of installation has produced devices with decreased serviceability. FIG. 6 depicts, schematically, a previous backflow preventor 110 which attempted to provide ease of serviceability by including both valves in 112a, 112b in a vertical configuration and a cover 114 which, when removed, permits access to the valves 112a, 112b (e.g., for maintenance purposes) in a vertical direction. The device shown in FIG. 6, however, provides a less than optimal performance. This is at least partially because, owing to the orientation of the valves 112a, 112b with respect to the inlet opening 116 and outlet opening 118 flow through the valve openings 116, 118 is forced to follow a divergent path (indicated by solid arrow streamlines 120a, 120b). The blocking action of the valve disks 122a, 122b, causing this divergent flow 120a, 120b, provides resistance to flow through the backflow preventor 110 and increases the pressure drop which the backflow preventor produces.
The device depicted in FIG. 6 also has deficiencies from the point of view of installation. In general terms, the cost of installation is least when the backflow preventor occupies the smallest amount of space. Thus, when a backflow preventor is installed in a building, it is desired to minimize the floor space required for installation. When the backflow preventor is installed outside a building, the expense of installation is related to the size of the enclosure required (e.g., enclosure 132 depicted in FIG. 7). When the backflow preventor is installed underground, it is desirable to minimize the size of the trench (not shown) required for underground installation.
As seen in FIG. 6, the inlet conduit and outlet conduit 124, 126 occupy a horizontal distance 128 which determines the minimum amount of space theoretically needed for installation of a backflow preventor. The upper portion 134 of the backflow preventor 110 occupies a horizontal extent 136 which is only slightly greater than theoretically minimum horizontal extent 128 required for installation. However, the lower portion 138 has a minimum horizontal extent 142 which is substantially greater, principally because the handle portions 144a, 144b of the shutoff valves extend outward from the housing 146 in a direction which is parallel to the axis of the conduits 124, 126 (i.e., parallel to a line passing through the conduits 124, 126). Moreover, an even larger horizontal expanse 148 is required to accommodate opening of the shutoff valves since the handles 144a, 144b move in a direction parallel to the axis of the conduits 124, 126.
FIG. 7 depicts another configuration for a backflow preventor which also has certain deficiencies. The axes 152a, 152b along which the first and second check valves 154a, 154b extend (defined, for these purposes, as a line passing through the center of the inlet port of the valves 154a, 154b and parallel to the direction of flow into the valves) are parallel and both extend at an angle of about 45.degree. to vertical. Access for maintenance is obtained by removing covers 156a, 156b to provide openings. The openings lie in planes 158a, 158b which are inclined to the horizontal by about 45.degree.. Because neither of the openings lies in a horizontal plane, the device does not provide for access in a vertical direction. This represents a drawback to the serviceability of the device in FIG. 7.
Installation of the device shown in FIG. 7 also has certain drawbacks. Installation requires certain additional parts such as 90.degree. elbows 162a, 162b to change the flow direction from the upward and downward flow of the inlet and outlet conduits 124, 126 to the horizontal flow direction of a backflow preventor 164. The size of the enclosure 132 required is relatively large to accommodate the extra parts 162a, 162b and since the two shutoff valves 166a, 166b and check valves 154a, 154b are generally linearly arrayed. Because of the change in flow direction, the flanges 168a, 168b for installing the backflow preventor 164 are vertically oriented. This requires provision of supports 172a, 172b for supporting and positioning the backflow preventor 164 at least during installation. As with the device depicted in FIG. 6, the check valves 154a, 154b of the device in FIG. 7 are of a type requiring that the flow through the valves be divergent 120a, 120b around the edges of the valve disks.
FIG. 8 depicts another type of previously-provided backflow preventor also having certain deficiencies.
The axes 152c, 152d, along which the first and second check valves 154a, 154b extend, are perpendicular and both extend at an angle of 45.degree. to vertical. Covers 156c, 156d cover access openings which lie in planes 158c, 158d, neither of which lies in a horizontal plane. Additional parts such as elbows 162c, 162d are required for installation. The two shutoff valves 166c, 166d and the two check valves 154c, 154d are generally linearly arrayed. The means for connection 168c, 168d of the inlet and outlet of the stop valves 166c, 166d are vertically oriented. The check valves 154c, 154d are of a type requiring that the flow through the valves be divergent 120a, 120b around the edges of the valve disks.
Typically, a check valve is designed to maintain its open configuration as long as there is flow through the valve. Once the flow stops or drops below a predetermined value, the check valve closes. Typically, check valves are designed so that, once the valve is closed, the inlet pressure must exceed a predetermined threshold before the valve will open. Usually, a single structure, typically a spring, is used both to provide the force to hold the valve closed (until the threshold is reached), and to provide the biasing force which moves the valve from the opened to the closed position. Because the biasing device provides some force tending to close the valve, even during normal flow conditions, a countervailing force must be provided to counteract the closing force and maintain the valve open, during normal flow conditions. Typically, the countervailing force is provided by the fluid moving through the valve. Accordingly, as the pressurized fluid moves through the valve, some amount of work is expended in holding the valve in the open position in opposition to the biasing force tending to close the valve. This expenditure of work causes a pressure drop across the check valve, so that the check valve itself necessarily creates a certain amount of loss of the pressure head. The amount of pressure minimally required at the inlet in order to maintain the valve in the open position is termed the "hold-open pressure." It is desirable to minimize the pressure drop or head loss during transit through the check valve, and, thus, it is desirable to reduce the hold-open force. Particularly, it is desirable that the hold-open force should be less than that from the threshold pressure.
Accordingly, a number of previous check valves having a biasing device have been produced, which create a greater force on the valve when it is in the closed position than when in the open position.
Many previous designs for reduced hold-open pressure check valves involve providing a linkage of one or more rigid pivoting arms connecting the clapper to the wall or body of the valve. U.S. Pat. No. 980,188, issued Jan. 3, 1911, to Blauvelt, for example, discloses a flap or swing-type valve having a clapper which can pivot toward or away from a valve seat. The clapper is pivotally connected to a rigid link or arm which, in turn, is pivotally connected to a spring.
Other valving devices include a knuckle or toggle-type linkage having two or more relatively pivoting arms or links.